Researchers in the area of family structures have devoted considerable attention to birth order and family size. In general, the findings of this research suggest that both of these structures affect parent-child relationships as well as certain behavioral and psychological outcomes of children. Although voluminous, these studies unfortunately suffer from major methodological problems. One of the most serious of these is the failure to include sibling spacing as a factor affecting both of these structures. The proposed research is based onthe premise that for a study to be accurately specified, it must include all three structures--number of siblings, sibling spacing, and birth order. Research by the present principal investigator suggests that not only is sibling spacing an important control variable, but that there may be a critical period for spacing. This finding holds at least for quality of parent-child relationships. The proposed study will investigate whether the sibling spacing findings also apply to intellectual and personality outcomes of children. Specifically, the proposed research will empirically examine: (1) The importance of sibling spacing in studies of family size and birth order, (2) Whether increases in spacing between siblings have a curvilinear relationship with the measures under investigation, and by implication, if there appears to be a critical period for spacing between siblings, (3) Whether the same spacing results apply to all categories of birth order and sibling size, (4) If sex of closest-spaced sibling makes a difference in the spacing results. The specific outcomes to be examined include: school achievement and cognitive performance; approval-dependency needs and affiliation; and self-esteem. These are believed to be reasonable success indicators. The study will employ multiple regression procedures on two large already-gathered sets of data. This methodology assures relatively quick, accurate, and inexpensive results; the use of two data sets allows replication. The findings should give direction for further research as well as have applied implications.